Radiation detectors are well known and are regularly used in various fields. Although originally developed for atomic, nuclear, and elementary particle physics, radiation detectors can now be found in many other areas of science, engineering, and everyday life. Some examples of the areas where radiation detectors are found are deep space imaging, medical imaging, e.g., positron emission tomography, tracking detection in high-energy physics, and radiation-trace imaging for the purpose of national security, among others. In experimental and applied particle physics and nuclear engineering, a radiation detector is a device used to detect, track, and/or identify high-energy particles such as those produced by nuclear decay, cosmic radiation, or particles generated by reactions in particle accelerators. In order to detect radiation, it must interact with matter; and that interaction must be recorded. The main process by which radiation is detected is ionization, in which a particle interacts with atoms of the detecting medium and gives up part or all of its energy to the ionization of electrons (or generation of electron-hole pairs in semiconductors). The energy released by the particle is collected and measured either directly, e.g., by a proportional counter or a solid-state semiconductor detector, or indirectly, e.g., by a scintillation detector. Thus, there are many different types of radiation detectors. Some of the more widely known radiation detectors are gas-filed detectors, scintillation detectors, and semiconductor detectors.
Gas-filled detectors are generally known as gas counters and consist of a sensitive volume of gas between two electrodes. The electrical output signal is proportional to the energy deposited by a radiation event or particle in the gas volume. Scintillation detectors consist of a sensitive volume of a luminescent material (liquid or solid), where radiation is measured by a device that detects light emission induced by the energy deposited in the sensitive volume.
Semiconductor detectors generally include a sensitive volume of semiconductor material placed between a positive electrode (anode) and a negative electrode (cathode). Incident radiation or particles are detected through their interactions with the semiconductor material, which create electron-hole pairs. The number of electron-hole pairs created depends on the energy of the incident radiation/particles. A bias voltage is supplied to the electrodes, causing a strong electric field to be applied to the semiconductor material. Under the influence of the strong electric field, the electrons and holes drift respectively towards the anode (+) and cathode (−). During the drift of the electrons and holes an induced charge is collected at the electrodes. The induced charge generates an electrical current which can be measured as a signal by external circuitry. Since the output signal is proportional to the energy deposited by a radiation event or particle in the semiconductor material, charge collection efficiency primarily depends on the depth of interaction of the incident radiation with the semiconductor material and on the transport properties, e.g., mobility and lifetime, of the electrons and holes generated. Thus, for optimal operation, e.g., maximum signal and resolution, of the detector, the collection of all electron-hole pairs, i.e., full depletion, is desirable.
One member of the semiconductor detector family is a Silicon (Si) Drift Detector (SDD) introduced by E. Gatti and P. Rehak in 1983 (Nucl. Instr. and Meth. A 225, pp. 608-614, 1984; incorporated herein by reference in its entirety). In the SDD, an additional electric field parallel to the surface of the wafer is added in order to force the electrons in the energy potential minimum to drift towards the n+ anode based on the principle of the sideward depletion. This is achieved by implanting two arrays of p+ electrodes on both sides of the wafer, instead of the single p+ implants. The p+ electrodes are suitably biased with a voltage gradient in order to provide an electric field parallel to the surface. Once generated by the ionizing radiation, the electrons are focused in the bottom of the potential channel and driven towards the anode region of the detector while the holes, driven by the depletion field, are quickly collected by the nearest p+ electrodes. In the region close to the collecting anode, the bottom of the potential channel is shifted towards the surface where the anode is placed, by suitably biasing the electrode on the opposite side.
The cloud of electrons induces to the anode an output pulse only when the electrons arrive close to it because the electrostatic shelf of the p+ electrodes. The drift time of the electrons may be used to measure one of the interaction coordinates while the collected charge allows the energy released by the incident ionizing event to be measured. SDD is characterized by a very low capacitance of the electrode collecting the signal charge independent of the active area of the device.
The spiral Si drift detector (SDD) is a special type of SDD family of detectors that utilize the cylindrical geometry exemplified in FIG. 1A or a square geometry used for best packing in space shown in FIG. 1B. FIG. 2 illustrates a cross section of the SDD from FIG. 1A. The SDD has a small anode 2 (small relative to a PIN diode anode) at one surface of the substrate 10 and an entrance window layer 6 at the opposite surface of the substrate. Use of a smaller anode results in lower capacitance and thus less of the undesirable electronic noise, resulting in improved resolution. The anode 2 can be surrounded by multiple doped rings 15. The doped rings 15 are biased in such a way that they result in an electric field which causes electrons to flow towards the anode 2. The doped rings 15 can have the same doping or conduction type as the entrance window layer 6. The doped rings 15 and the entrance window layer 6 can have the opposite doping or conduction type as the substrate 10. However, usually the doped rings 15 and the entrance window layer 6 are more highly doped than the substrate 10. The anode 2 can have the same doping or conduction type as the substrate 10. However, usually the anode 2 is more highly doped than the substrate 10.
As shown in FIG. 2, one voltage bias V1 can be applied to the innermost doped ring that is closest to the anode 2 and another voltage V2 can be applied to the outermost ring. Because the rings are electrically coupled, the voltages at the innermost and outermost rings can create a voltage gradient across all of the rings. Another voltage V3 can be applied to the entrance window layer 6. The voltage applied to the entrance window layer V3 can be similar in magnitude to the voltage V2 on the outermost ring. The voltage on the innermost ring V1 can have a lower absolute value than the voltage at the outermost ring V2 or at the entrance window V3. Due to the voltage gradient across the rings and the voltage applied to the entrance window 6, the charge carrier can be drawn towards the anode 2. If V2 and V3 are more negative than V1 and V1 is more negative than the anode 2, then an electron cloud resulting from impinging radiation can be directed to the anode. Although the prior art SDDs can have reduced electronic noise compared with the prior art PIN diode, such SDDs with electrically coupled rings can be costly to manufacture.
In a spiral SDD, the ion implants are needed as both the rectifying junction and voltage divider to create a potential gradient (or drift field) in the SDD for carriers generated by incident particles to drift to the collection anode, as shown in FIG. 3 (illustrating a double-sided spiral SDD as described by P. Rehak et al., IEEE Trans. Nucl. Sci., Vol. 36, No. 1, 203-209 (1989); P. Rehak et al., Nucl. Instr. and Meth. A, 624, 260-264 (2010); W. Zhang et al., IEEE Trans. Nucl. Sci., Vol. 47, No. 4, 1381-1385 (2000); E. Gatti and P. Rehak, Nucl. Instr. and Meth. A, 225, 608 (1984); each of which is incorporated herein by reference in its entirety).
The optimum surface potential profile Φ(r) on the front surface that gives a minimum drift time of electrons had been approximated by P. Rehak et al. (1989) for a uniform backside bias voltage of VB when the radius of the spiral SDD (R) is much larger than the detector thickness (d), i.e., R>>d. The advantage of the spiral as voltage divider is that it can be easily designed and fabricated with non-constant pitch (p(r)) and width (W(r)), which give a largely exotic surface potential distribution that guaranties a minimum drift time of carriers. To do the same with an external resistor chain, one may have to choose resistors varying from hundreds of kΩ's to MΩ's of nearly random values. In general, a double-sided spiral SDD, as shown in FIG. 3 can potentially have a better surface field distribution needed for the minimum drift time (see FIG. 4). However, since the rectifying junction and voltage divider are coupled together in such spiral SDD designs, there is a constraint on the ratio of spiral width to pitch (W(r)/p(r)) for more uniform drift field in the drift channel, which results in a large current IS for a given voltage difference (Vout−VE1) between the start and end of the spiral. Also, the heat generated by the spiral stays with the SDD, which may cause problems in cooling down the detector. Furthermore, and most importantly, for a spiral SDD array of N×M, the power consumption can be substantial, approaching Watts, e.g., 1-5 W, for large arrays (>100 elements) based on P=N×M×(Vout−VE1)×IS. Therefore, there is a continuing need to develop new SDD systems and SDD arrays that would avoid overheating, high power consumption, and limitations in the uniformity of the drift field.